doc: more go1.8.html tweaks and new context additions section

TBR=See https://golang.org/cl/33244

Updates #17929

Change-Id: Id5d5472cf1e41472d8d0f82ee133c7387257ba2b
Reviewed-on: https://go-review.googlesource.com/33664Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>